Defines an instant in the datetime continuum.
This interface expresses the datetime as milliseconds from 1970-01-01T00:00:00Z.

The implementation of this interface may be mutable or immutable.
This interface only gives access to retrieve data, never to change it.

Methods in your application should be defined using ReadableInstant
as a parameter if the method only wants to read the instant without needing to know
the specific datetime fields.

The compareTo method is no longer defined in this class in version 2.0.
Instead, the definition is simply inherited from the Comparable interface.
This approach is necessary to preserve binary compatibility.
The definition of the comparison is ascending order by millisecond instant.
Implementors are recommended to extend AbstractInstant instead of this interface.